Method for the cold extrusion of bolts, in particular for fastening the cutting blades of mowers or the like

ABSTRACT

A method for producing metal parts such as special bolts ( 20 ) by means of cold extrusion in pressing dies comprises a first step of cutting to size a metal blank ( 30 ) to be modelled with predetermined dimensions, at least one step of cold pressing in a die in order to reduce one or more portions of the blank ( 30 ) to predetermined diameters, and a step of cold pressing in a die during which a coupling key ( 25 ) is formed on a portion ( 32 ) of the blank ( 30 ), the lateral surface ( 36 ) of the key protruding beyond the lateral surface of this portion ( 32 ), with respect to the longitudinal axis of the blank ( 30 ). The resulting product can be used in particular as a special bolt for the positive coupling of a mowing machine blade to a rotating plate in order to rotate the blade.

TECHNICAL FIELD

This invention refers to a method for the cold extrusion of bolts.

In particular, this invention refers to a method for the cold extrusionof a bolt equipped with a key or lug for coupling the bolt to rotatingmechanical parts such as, for example, cutting blades of mowing machinesor the like.

The invention also refers to a bolt obtained by means of this method.

This invention can be applied in the field of the mechanical industry.

BACKGROUND ART

During the assembly of rotating mowing machines, special bolts are oftenused in order to rigidly fix the cutting blades to a rotating plate inturn connected to the motor of the machine.

An example of this nature is illustrated in FIG. 1, which shows a pairof bolts 10, 10′, a pair of blades 50, 50′ and a rotating plate 60belonging to a mowing machine known to the background art.

These special bolts comprise a first threaded portion, designed to becoupled to a corresponding blocking nut with the possible interpositionof a washer, a second portion equipped with a key (or lug) for centeringand positioning the bolt with respect to the rotating plate, a thirdportion, whose diameter corresponds to the diameter of the hole in theblade, and a head designed to press against the body of the blade.

Special bolts of the type described above are generally obtained bymeans of cold extrusion methods on multipress machines, starting fromblanks of steel bars placed inside appropriate pressing dies.

In this context, it should be noted that in the solutions known to thebackground art, the key (or lug) is obtained by caulking after thepressing of the body of the bolt.

In practice, the bolt body is clamped inside a caulking machine whichacts on a lateral portion of the bolt shank in order to form a pair ofsymmetrical key elements that rise from the bolt shank, forming acorresponding pair of notches within the shank beside the key elements.

This solution, well known to the background art, is illustrated in FIGS.2 and 2 a which show a special bolt 10 with a first threaded portion 11,a collar 12, a head 13, a central portion 14 with a respective pair ofsymmetrical key portions 15, 15′ and a pair of notches 16, 16′ each ofwhich is positioned alongside a respective key portion 15, 15′.

The key (or lug) obtained by caulking projects radially from the side ofthe central portion 14 of the bolt 10, and plays an important role ofanti-unbolting in the mechanical coupling of the blades to the rotatingplate.

It must, in fact, ensure positive coupling of the bolt/blade assembly tothe plate and the key must therefore be completely reliable in terms ofmechanical features in order to guarantee the safety of those using themowing machines.

The technical solution described above presents a number of drawbacksand disadvantages.

From the technical-economic point of view, the caulking operation whichleads to the formation of the key (or lug) on the body of the specialbolt represents an additional operation with respect to the extrusion ofthe bolt body and which requires a specialised machine for thisoperation alone.

This therefore leads to an additional cost.

Furthermore, currently available caulking machines are not able toguarantee that all the machined bolts are equipped with a key that isgeometrically constant and always protrudes beyond the side of the boltbody.

In other words, the caulking machines known to the background art cannotensure that the positive coupling required between the bolt/blade systemand the rotating plate of the mowing machine is always obtained or isconsistently obtained.

In addition, the operation of flattening the key by caulking does notfollow the direction of the steel fibres and tends to create surfacetension and fragility up to the point of triggering microfissuresbetween the surface of the shank diameter and the caulked key, these,microfissures being difficult to verify and thus increasing the risk ofpossible bolt fixing grip problems.

The denser hatched section in the upper part of FIG. 2 a illustrates thecompression of the fibres in the caulked area.

Moreover, the mechanical stress to which the steel of the bolt body issubjected during caulking and formation of the two symmetrical keyportions is considerable, and cases have occurred in which the keyportions snapped off during caulking or, even worse, during the use forwhich these bolts are designed.

These aspects are extremely important from the safety point of view,since a failed positive coupling between the bolt/blade unit and therotating plate can lead to an inopportune rotation of the blades andeven to detachment of the blades from the rotating plate during use ofthe mowing machine, with possible serious consequences for the user andanyone close to the machine.

DESCRIPTION OF THE INVENTION

This invention proposes to overcome the drawbacks and disadvantagestypical of the background art, and thus to provide a method for theproduction of special bolts, in particular for mowing machines, thatallows the mass production of special bolts equipped with a built-in key(or lug) for coupling with other mechanical elements, the key beingobtained directly by cold pressing and whose presence is guaranteed onall the pieces produced.

From a careful metallographic analysis of the arrangement of the fibresit can be seen that the key obtained by cold extrusion directly in thedie is an integral part of the bolt shank. Therefore, appropriatelydesigned radiuses are present between the diameter and the key, makingthe future coupling of the key superior in terms of quality with respectto the background art.

This is achieved by implementation of the features described in the mainclaim.

The dependent claims describe advantageous embodiments of the methodaccording to the invention.

This invention also proposes to provide a special bolt, in particular abolt designed to be used for coupling mechanical pieces such as theblades and rotating plate of a mowing machine.

This special bolt is described in the claim and its dependent claims.

DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become evident onreading the following description of a form of embodiment of theinvention, given as a non-binding example, with the help of the encloseddrawings, in which:

FIG. 1 is an exploded diagrammatic prospective view of the blade fixingarea of a mowing machine known to the background art;

FIG. 2 is a front view of a special bolt known to the background art, inparticular for the coupling of a blade/rotating plate unit of a mowingmachine;

FIG. 2 a is a cross-section of the special bolt in FIG. 2, correspondingto the central portion of the bolt shank;

FIG. 3 is a front view of a special bolt according to the invention;

FIG. 3 a is a cross-section of the special bolt in FIG. 3, correspondingto the central portion of the bolt shank; and

FIGS. 4 a to 4 e show the side views of the various steps of the methodaccording to the invention for obtaining a bolt shown in FIG. 3.

DESCRIPTION OF ONE EMBODIMENT

In FIGS. 3 and 3 a, a special bolt 20, in particular a bolt designed tobe used for coupling a blade to a rotating plate of a mowing machine,consists of a first threaded portion 21, a collar 22, a head 23 and acentral portion 24 with a built-in key (or lug) 25 whose lateral surface36 (see FIGS. 4 d and 4 e) protrudes externally with respect to thelateral surface of the central portion.

As already mentioned, the purpose of this built-in key (or lug) 25 is toprovide a means of positive coupling between a blade/bolt unit and therotating plate of a mowing machine.

The special bolt 20 is obtained by means of a cold extrusion process ona multipress, and the piece emerging from the last pressing die issubjected to a final roiling to produce the threading 21, withoutundergoing caulking of the key 25 as in the solutions know to backgroundart.

In particular, FIG. 4 a shows a steel bar blank 30 cut to size and readyfor insertion in a first cold pressing die of a multipress machine.

FIG. 4 b shows the result of the first pressing operation, during whichthe first portion 31, designed to be subsequently threaded, is formed.

In a second extrusion operation (FIG. 4 c) a central portion 32 isformed, with a smaller diameter than the upper portion 33.

During the next step (FIG. 4 d) the central portion is further dividedinto two parts 32 and 34, and the key (or lug) is formed at the sametime, with its lateral surface 36 protruding with respect to the lateralsurface of the portion 32.

In the last extrusion step (FIG. 4 e) the complete special bolt isformed, as also shown in FIG. 2.

The formation of the key (or lug) 25 represents a delicate step in thebolt production process.

In order to prevent an untimely breakage of the pressing dies, it is infact necessary to carefully monitor certain design parameters, such asfor example the roughness of the equipment, the radius between the boltshank and the key (or lug) and the geometric dimensions of the keyitself.

The end result is, however, a bolt obtained by cold extrusion and whichpresents a built-in key (or lug) without requiring any further caulkingoperations.

At the same time, the method according to the invention guarantees thepresence of the key (or lug) in: all the pieces produced since they mustnecessarily pass through a die which gives them their final shape, thusraising the quality level of the production with respect to the knownmethods.

The invention is described above with reference to a preferredembodiment.

It is nevertheless clear that the invention is susceptible to numerousvariations that lie within its scope, in the framework of technicalequivalents.

By way of example, it is clear that the materials used to produce thespecial bolts according to the invention can consist of various metalsand their alloys.

Furthermore, although the invention is described with particularreference to bolts that can be used to couple blades and rotating platesof mowing machines, it is clear that the method according to theinvention can be implemented for the production of bolts equipped withcoupling keys with two or more extrusion diameters designed to achieve apositive coupling of pairs of any mechanical pieces, such as for examplegear shafts, etc.

1. A method for producing special bolts by means of cold extrusion inpressing dies, comprising: cutting to size, a metal blank to be modelledwith predetermined dimensions; cold pressing in a die in order to reduceone or more portions of the blank to predetermined diameters; and colderpressing in a die during which a coupling key is formed on a portion ofthe blank, wherein the lateral surface of the key protrudes beyond thelateral surface of the portion of the blank, with respect to thelongitudinal axis of the blank.
 2. The method of claim 1, furthercomprising the step of cold pressing in a die, during which a collar anda head above the collar are formed on the blank.
 3. The method of claim1, wherein at least one portion of the blank is subjected to rolling inorder to produce the treading, following the steps of cold pressing indies.
 4. The method of claim 1, wherein the blank is made from metal. 5.The method of claim 4, wherein the die is made from steel or widia.
 6. Ametal element for coupling mechanical pieces comprising: a first portionwith a first predetermined diameter; a second portion with a secondpredetermined diameter larger than the first diameter; and a thirdportion with a third predetermined diameter larger than the seconddiameter, wherein the second portion presents a key or lug whose lateralsurface protrudes beyond the surface of the second portion with respectto the longitudinal axis of the metal element.
 7. The metal element ofclaim 6, wherein the element further comprises a head positioned abovethe third portion.
 8. The metal element of claim 6, wherein the firstportion is threaded.
 9. The metal element of claim 8, wherein the metalelement is a special bolt for the positive coupling of a blade of amowing machine to a rotating plate in order to rotate the blade.
 10. Themethod of claim 2, wherein at least one portion of the blank issubjected to rolling in order to produce the threading, following thesteps of cold pressing in dies.
 11. The method of claim 2, wherein theblank is made from metal.
 12. The method of claim 3, wherein the blankis made from metal.
 13. The method of claim 10, wherein the blank ismade from metal.
 14. The element of claim 6 wherein the element isproduced by means of a cold extrusion method in a die according toclaim
 1. 15. The element of claim 6 wherein the element is produced bymeans of a cold extrusion method in a die according to claim
 2. 16. Theelement of claim 6 wherein the element is produced by means of a coldextrusion method in a die according to claim
 3. 17. The metal element ofclaim 7, wherein the first potion is threaded.